Malignancy is one of the major diseases that seriously threaten the human life and the quality of life. In recent years, chemotherapy has made considerable progress, significantly prolonged the survival of cancer patients, and in particular, made significant breakthroughs in the treatment of leukemia, malignant lymphoma. However, satisfactory results in treating solid tumors (including lung cancer, liver cancer, bladder cancer, and colon cancer, etc.), which most seriously endanger human life and health, and account for more than 90% of the malignancies, have not been achieved.
With the rapid developments in the life sciences, tumor mechanism has been gradually clarified, new ideas have appeared successively, such as inhibiting tumor growth factor, intervening cancer signaling, inhibiting tumor angiogenesis, and inducing tumor cell apoptosis. However, since the pathogenesis of malignant tumors is very complex, clinical practice in recent years has shown that the better clinical efficacy can be achieved only by using them in the combination with cytotoxic medicaments. Therefore, it is of great significance to use key enzymes relevant to the proliferation and differentiation in the tumor cells as a drug target for developing novel medicaments which have high efficiencies, low toxicities as well as strong specificities, and can selectively act on specific target(s).
Topoisomerase I (Top1) has become one of the key target enzymes for designing novel anti-cancer medicaments. The enzyme is necessary for cell survival and involves in the whole procedure of DNA replication, transcription, recombination, and repair; and the Top1 content in a variety of tumor cells, especially lung cancer, stomach cancer, colon cancer, and ovarian cancer, etc., is significantly higher than that in normal cells. Therefore, Top1 inhibitors have not only high efficacies and broad anti-tumor spectra, but also good selectivities for tumor cells. The Top1 inhibitors have been classified as one of six categories of key anticancer medicaments by the U.S. National Cancer Institute.
In all kinds of Top1 inhibitors, camptothecin (CPT) compounds, the most classical Top1-specific inhibitor, have been studied most deeply and become one highlight of the anticancer medicaments research in recent years. In these compounds, Irinotecan (CPT-11) and Topotecan have been successfully marketed, used for the treatment of metastatic colorectal cancer and refractory ovarian cancer etc., and achieved good efficacies. Therefore, the camptothecin derivatives, paclitaxel and retinoic acid compounds have been praised as the three major findings of anti-cancer medicaments in the 1990s. However, such compounds also have prominent problems as follows: 1) the metabolically labile in vivo, that is, the E-ring lactone structure necessary for activity is hydrolyzed into a carboxylic acid salt form in the human body very quickly, and the form of carboxylic acid salt is not only ineffective to Top1, but also more likely to bind human serum albumin; 2) Top1 cleavable-complex (Top1cc) need to be maintained relatively long time to be converted into DNA damage. However, camptothecins tend to dissociate from Top1cc, therefore, the period for intravenous dripping must be prolonged when the camptothecin medicaments are used in clinic; 3) camptothecin possesses strong hydrophobicity due to the unique five-ring conjugated planar structure, resulting in poor water solubility; 4) camptothecins have certain toxic side effects, such as leukopenia, nausea, vomiting, etc., thus restricting the safe dose and in turn the efficacy; 5) resistance. At present, several camptothecin resistant Top1 mutants have been reported. The most frequent mutations, such as Asn722, Arg364, etc., can result in the resistance to camptothecin.
Recently, non-camptothecin Top1 inhibitors have become the hotspot in the research of anti-cancer medicaments. Indolocarbazole compounds are studied deeply at present, wherein, J-107088 (Edotecarin) has entered into the clinical study. However, studies have shown that these compounds are not specific Top1 inhibitors, and also have the inhibitory activity for protein kinase C or checkpoint kinase Chk-1. Luotonine A and Lamellarin D are also Top1 inhibitors isolated from nature in recent years, but they have defects, such as complex structures, poor specificities, and high toxicities. At present only camptothecin anti-tumor medicaments are used widely in clinical practice.
In non-camptothecin Top1 inhibitors, indenoisoquinoline compounds increasingly attracted people's attention. In these compounds, NSC 314622 was initially synthesized by Cushman group (Purdue University) in 1978 and was found to have strong anti-tumor activity. However, its mechanism of action remains unclear. Until 1998, Pommier et al of NCI found that Top1 is the target site of indenoisoquinoline antitumor compounds. This result promoted the in-depth study of such compounds. Cooperated with NCI, Cushman group synthesized four to five hundred indenoisoquinoline compounds, comprehensively studied these compounds for their Top1 inhibitory effect as well as the anti-tumor activities, elucidated the structure-activity relationship of these compounds and obtained the crystal structure of one such compound with Top1-DNA covalent complex. Among the indenoquinoline compounds, NSC 706744, NSC 725776 and NSC 724998 showed a prominent antitumor activities in vitro and in vivo, and Top1 inhibitory activities, of which the latter two have entered the NCI's clinical research.

Comparing with camptothecins, indenoisoquinolines have several unique advantages: 1) the chemical structures of such compounds are very stable, while camptothecin molecule contains hydroxy lactone ring which is readily hydrolyzed; 2) NSC 314622 can cause Top1 mediated DNA cleavage, but the resulted cleavage sites are different from CPT, which may contribute to target different action sites on the genome, therefore, it is likely to find effective anti-tumor medicaments having treatment spectra different from camptothecins; 3) a ternary complex formed from these compounds with the Top1-DNA complex is more stable. The cleavage complex formed from camptothecins is reversible, therefore it is necessary to takes a long time for intravenous dripping to better exert anti-tumor activity. The advantage of these compounds can overcome this problem; 4) these compounds help to overcome the shortcomings of camptothecins such as resistance, poor water-solubility, and so on.
The research experience of the topoisomerase II and tubulin inhibitors has showed that the anticancer drugs having same mechanism of action but different skeletons could have different anti-tumor spectra. Therefore, it is urgent to develop novel non-camptothecin Top1 inhibitors with new skeletons, high efficiencies and low toxicities.